Animate object on arc trajectory
Like in this answer https://blender.stackexchange.com/a/5521/1853 Change the origin of the object where you need it and rotate the object using the new pivot point.



To animate, go the frame where you want the animation to start and press I to create a key frame for the rotation. Move then to a different frame, rotate your object and press I to create a new key frame. 



To swing the hammer in an arc motion change the origin further away from the handle.